Terminate WebTransport sessions when unloading page

Fixes w3c/webtransport#127.

source

@@ -4080,6 +4080,18 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://w3c.github.io/manifest/#dfn-processing-a-manifest">process the manifest</dfn></li>
     </ul>
    </dd>
+
+   <dt>WebTransport</dt>
+   <dd>
+    <p>The following features are defined in <cite>WebTransport</cite>: <ref spec=WEBTRANSPORT></p>
+
+    <ul class="brief">
+     <li><dfn data-x="webtransport-session-terminate" data-x-href="https://w3c.github.io/webtransport/#session-terminate">terminate</dfn></li>
+     <li><dfn data-x="webtransport-state" data-x-href="https://w3c.github.io/webtransport/#webtranport-state">[[State]]</dfn></li>
+     <li><dfn data-x="webtransport-session-slot" data-x-href="https://w3c.github.io/webtransport/#webtransport-session-slot">[[Session]]</dfn></li>
+     <li><dfn data-x-href="https://w3c.github.io/webtransport/#webtransport"><code>WebTransport</code></dfn></li>
+    </ul>
+   </dd>
   </dl>
 
   <hr>
@@ -88498,6 +88510,17 @@ dictionary <dfn dictionary>PageTransitionEventInit</dfn> : <span>EventInit</span
      <li><p>Empty <var>window</var>'s <span>list of active timers</span>.</p></li>
     </ol>
    </li>
+
+   <li>
+    <p>For each <code>WebTransport</code> object <var>transport</var> whose <span>relevant global
+    object</span> is <var>window</var> and <span data-x="webtransport-state">[[State]]</span>
+    is <code data-x="">"connected"</code>, <span
+    data-x="webtransport-session-terminate">terminate</span> <var>transport</var>'s
+    <span data-x="webtransport-session-slot">[[Session]]</span>.</p>
+
+    <p><span class="XXX">This needs to be done in workers too. See
+    <a href="https://github.com/whatwg/html/issues/6831">#6831</a>.</span></p>
+   </li>
   </ol>
 
   </div>
@@ -124968,6 +124991,9 @@ INSERT INTERFACES HERE
    <dt id="refsWEBMCG">[WEBMCG]</dt>
    <dd><cite><a href="https://www.webmproject.org/docs/container/">WebM Container Guidelines</a></cite>. The WebM Project.</dd>
 
+   <dt id="refsWEBTRANSPORT">[WEBTRANSPORT]</dt>
+   <dd><cite><a href="https://w3c.github.io/webtransport">WebTransport</a></cite>, Bernard Aboda, Victor Vasiliev, Yutaka Hirano. W3C.</dd>
+
    <dt id="refsWEBVTT">[WEBVTT]</dt>
    <dd><cite><a href="https://w3c.github.io/webvtt/">WebVTT</a></cite>, S. Pieters. W3C.</dd>